How to precisely set orthographic scale
Unfortunately the answer by Duarte Farrajota Ramos does not address the orthographic scaling in imperial units. However, after pulling my hair out for a few hours, I finally understood that the Blender cameras do not respect the scene units (is this a bug?). So what I did was to set the orthographic scaling to 0.9144 so that it would match a yard (it seems that a yard is a better target than a foot) and then adjust the unit scaling in the scene properties, and voila, it works!